/Engine/Source/Runtime/Foliage/Public/FoliageType.h
struct FFoliageVertexColorChannelMask
uint32: 1
InvertMask
When unchecked, foliage instances will be placed only when the vertex color in the specified channel(s) is above the threshold amount.
float
MaskThreshold
Specifies the threshold value above which the static mesh vertex color value must be, in order for foliage instances to be placed in a specific area
uint32: 1
UseMask
When checked, foliage will be masked from this mesh using this color channel